1. Field of the Invention
The present invention relates to thermal printers and, more particularly, to fusing dye images in a receiver produced by such thermal printers.
2. Description of the Prior Art
Currently thermal dye transfers are usually followed by a fusing step to further "set" dye into the receiver. The term "thermal dye transfer" refers to all methods of transferring dye by thermal methods irregardless of whether the thermal energy is directly or indirectly generated and/or delivered, such as, but not exclusively resistive head, resistive ribbon, laser and ultrasonic thermal dye transfer. There generally are two technologies which are available for fusing. The first and most common is a thermal fusing process which involves reheating the receiver after thermal dye transfer. Because this technique uses thermal energy and generates a large amount of heat, generally a separate unit isolated from the heat sensitive donor is required to perform this operation. This then requires a distinct two-step process and two separate units, one for image transfer and one for fusing which in turn increases time and costs of thermal imaging. Such heat fusing steps involve the possibility of damage to the receiver in the process of heat treating it to fix or fuse dyes.
Solvent fusing can eliminate the problem of damage to the receiver and also possible damage to the dye caused by subsequent heating steps. In heretofore solvent fusing steps a receiver with a dye image transferred by thermal printing is placed in an enclosure adjacent to an open bath of solvent liquid. The liquid solvent vaporizes and this vapor impregnates the receiver and fuses the dye image into it.
In this method of solvent fusing, the solvent vapor concentration is dictated by the saturated vapor pressure of solvent at the ambient temperature. Sometimes, depending on the solvent being used, sufficient concentration can be reached which causes damage to the dye image. Another problem with this method is that with some solvents it is difficult to reach the appropriate concentration level to cause the solvent to impregnate the receiver to a sufficient extent so as to properly fix the dye image in the receiver. A significant quantity of solvent vapor may escape primarily during placement and removal of the receiver with respect to the enclosure. When the receiver layer is positioned inside the enclosure, the solvent liquid-vapor equilibrium is lost due to loss of vapor. As solvent liquid evaporates to re-establish equilibrium concentration, the liquid is cooled by evaporative cooling which results in a lower vapor concentration than the original, until such time that the liquid has absorbed sufficient heat from the surroundings to again reach ambient temperature. As a result, this natural vapor-liquid equilibrium method of solvent fusing is substantially unregulated or uncontrollable as it is affected by many variable factors including frequency of use, amount of vapor lost during receiver loading/unloading, liquid volume, vapor space volume, and construction material and configuration of enclosure.
Many solvents which are useful for fusing thermally transferred dye images into a receiver layer are toxic and flammable in varying degrees. Problems arising from uncontrolled leakage of solvent vapors may include personnel injury and contamination of adjacent processes as well as emission of solvent to the environment.